1. Field of the Invention
The present invention generally relates to a sheet feeding apparatus and an image forming apparatus, and more particularly to a feeding apparatus for separating and thus feeding sheets having high adhesiveness between the sheets.
2. Description of the Related Art
An image forming apparatus such as a copying machine and a printer conventionally includes a sheet feeding apparatus that sequentially feeds sheets stacked in a sheet stacking portion one by one from the uppermost sheet and thereafter feeds the sheets to an image forming portion.
In this type of sheet feeding apparatus, in the case of consecutively feeding the sheets, cut sheets are used, and these cut sheets are normally limited to free sheets and plain paper designated by a copying machine maker. Further, there have hitherto been adopted a variety of separating systems for surely separating these sheets one by one and thus feeding the sheets. This type of separating system may, for example, be a separating pad system for preventing double feeding by making a friction member abut, e.g., a feed roller with a predetermined pressure.
Another separating system is a retard type separating system. This system is one in which a separating portion is constructed of a feed roller rotating in a sheet feeding direction and a separating roller driven with a predetermined torque in a direction reversed to the sheet feeding direction and abutting the feed roller at a predetermined pressure. This separating portion permits passage of only the uppermost sheet of a sheet stack fed out by a pickup roller, and returns other sheets fed out following the uppermost sheet to the sheet stacking portion, thereby preventing the double feeding.
Herein, for surely separating and thus feeding the sheets by these separating systems, in the case of, for example, the retard type separating system, the sheets can be surely separated one by one in a way that optimizes a return torque and a pressurizing force of the separating roller by taking account of a friction force of the should-be-fed sheet.
By the way, with variety of sheet types (recording mediums), there are increasingly demands for forming images on sheets such as a coated sheet etc, of which the surface is subjected to a coating treatment in order to exhibit a whiteness degree and luster in response to a color-oriented market request in addition to super-thick sheets (carton boards), OHP sheets, art films and so on.
In the case of feeding the super-thick sheet, however, the super-thick sheet can not be picked up because its dead weight will resist conveying, and there is some fear that a jam will occur. Moreover, in the sheets made from an easy-to-charge resin material as in the case of the OHP sheet and the art film, the sheet surface is gradually becomes electrically charged due to friction between the sheets on the occasion of a feeding operation under a low-humidity environment, and will adhere to each other by dint of a Coulomb force. Hence, there is some fear that these sheets can not be picked up, and the double feeding occurs.
Further, the coated sheet of which the surface is coated with a coating substance composed of a coating material etc has a property that the sheets are adsorbed to each other in the case of being stacked under a high-humidity environment. Therefore, the coated sheets can not be picked up, and double feeding frequently occurs.
In the case of such a special type of sheets, the friction force itself between the sheets is equal to or smaller than that of the plain paper etc. However, the sheets are absorbed to each other by a much higher force than the friction force between the sheets with adsorbability (adhesion) caused by triboelectric charging under the low-humidity environment in the case of the resin material sheet and with the adsorbability under the high-humidity environment in the case of the coated sheet, and hence the sheets can not thoroughly be separated in the conventional separating systems. Namely, the conventional separating systems take account of only the friction force between the sheets, so that sheets can not be surely separated from each other when the adsorbability factors other than the friction force are present.
Such being the case, Japanese Patent Application Laid-Open Application No. H03-211136 proposes a technology for dissipating such high adsorbability between the sheets. This technology is that the sheets are previously raveled by blowing the air from the side surface (side end) of the sheet stack, and the adsorption (adhesion) between the sheets is vanished, in which state the sheets are picked up sheet by sheet from the upper sheet, and the separating portion provided downstream separates the sheets on a sheet-by-sheet basis. The sheet feeding apparatus using such a separation feeding system is adopted in a printing industry and in some of copying machines.
Herein, the reason why the air is blown is that the water content of the sheet is evaporated from the air flow between the sheets adsorbed to each other under the high-humidity environment, and the sheet is dried, thus reducing the adsorbability (adhesion). Accordingly, a raveling effect further rises when the air is hot air.
Then, in the separation feeding system including a portion (which will hereinafter be referred to as an auxiliary raveling portion) for blowing the air from the side surface of the sheet stack, the adsorption between even the sheets having the high adsorbability as described above can be dissipated by raveling the sheets in advance of feeding. Hence, separating performance is remarkably improved as compared with the already-described systems that simply utilize the friction force.
FIG. 11 is a view showing a construction of the conventional sheet feeding apparatus including the auxiliary air raveling portion. The construction and a function of this sheet feeding apparatus 100 will be explained. The sheets S are stacked in a sheet feeding deck 101, and the air supplied from a centrifugal separation type separating fan 102 is discharged obliquely upward from a raveling nozzle 103a at a predetermined wind speed and is thus blown into the side end of the sheets S.
Then, the air discharged obliquely upward from the raveling nozzle 103a enters the upper portion of the stack of sheets S, whereby some sheets, including the uppermost sheet S1, are floated. Hereafter, the uppermost floating sheet S1 is adsorbed onto a conveying belt 108. This conveying belt 108 is an endlessly-shaped belt composed of rubber and formed with a plurality of round holes 108a in predetermined positions, and is looped around two roller pairs 104, 105. Then, the sheet is adsorbed by a suction force of a centrifugal separation type suction fan 107 in a suction chamber 106 disposed in this conveying belt 108.
Herein, at this time, other than the uppermost sheet S1, a sheet S2 under the sheet S1 might be adsorbed. Therefore, the air discharged at a predetermined wind speed along the conveying belt 108 from a separating nozzle 103b also flows in between the adsorbed sheets S1 and S2, thereby peeling off the adsorbed sheets S1 and S2.
Next, when an adhesion sensor 109 detects that the uppermost sheet S1 is adsorbed onto the conveying belt 108, a drive roller 104 rotates in an arrowhead direction, whereby the sheet S1 is conveyed. Then, hereafter, the sheet S1 is pinched and conveyed by a draw-out roller pair 110, and, when a sheet feeding sensor ill detects a leading end of the sheet S1, the conveying belt 108 is stopped, and a negative pressure within the suction chamber 106 is canceled. With this operation, the sheet S1 is consecutively conveyed by the draw-out roller 110.
When the sheet S1 continues to be conveyed, and, when a sheet height detection sensor 112 detects that the uppermost sheet S1 reaches an incapable-of-adsorbing position by the conveying belt 108, a sheet tray 101a rises up to a predetermined position. A heater 113 is disposed on the air intake side of the separating fan 102, and the air from the separating fan 102 is heated by the heat generated in the heater 113 and is discharged toward the sheet S1 from the raveling nozzle 103a and the separating nozzle 103b. 
In this type of conventional sheet feeding apparatus and in the image forming apparatus including this sheet feeding apparatus, however, other than blowing, against the sheet S1, the hot air heated by the heat generated by the heater 113, an unillustrated heater is provided within the sheet feeding deck 101 in order to improve separating performance. Then, the sheets S are warmed up by this heater in a state of being stacked in the sheet feeding deck 101, thereby restraining the adsorbability between the sheets S.
Namely, when stacked in the sheet feeding deck 101, the sheets S are warmed up by the unillustrated heater, and, when the sheets S get floating by the raveling nozzle 103 and when the sheets S are separated by the separating nozzle 103b, the hot air is blown against the sheets. This contrivance greatly reduces the adsorbability between the sheets S.
In the case of reducing the adsorbability between the sheets S by use of the two heaters such as the heater 113 and the unillustrated heater, however, though the sheets can be surely separated, the apparatus becomes complicated, and besides the electric power consumption increases.